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64-Bit Compiler

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After you convert your 32-bit driver source code to use the new data types, you can use the 64-bit compiler to identify any type-related problems that you initially missed. The first time you compile this code for 64-bit Windows, the compiler might generate many pointer-truncation or type-mismatch warnings. Use these warnings as a guide to make your code more robust. It is good practice to eliminate all warnings, especially pointer-truncation warnings.

The following is an example of such a warning:

warning C4311: 'type cast' : pointer truncation from 'unsigned char *' to 'unsigned long '

For example, the following code can generate the C4311 warning:

buffer = (PUCHAR)srbControl;
(ULONG)buffer += srbControl->HeaderLength;

To correct the code, make the following changes:

buffer = (PUCHAR)srbControl;
(ULONG_PTR)buffer += srbControl->HeaderLength;

Predefined macros

The compiler defines the following macros to identify the platform.

Macro Meaning

_WIN64

A 64-bit platform.

_WIN32

A 32-bit platform. This value is also defined by the 64-bit compiler for backward compatibility.

_WIN16

A 16-bit platform.

The following macros are specific to the architecture.

Macro Meaning

_M_IA64

A 64-bit Intel platform.

_M_IX86

A 32-bit Intel platform.

Do not use these macros except with architecture-specific code. Instead, use _WIN64, _WIN32, and _WIN16 whenever possible.

64-Bit compiler switches and warnings

There is a warning option to assist porting to 64-bit Windows. The -Wp64-W3 switch enables the following warnings:

  • C4305: Truncation warning. For example, "return": truncation from "unsigned int64" to "long."

  • C4311: Truncation warning. For example, "type cast": pointer truncation from "int*_ptr64" to "int."

  • C4312: Conversion to bigger-size warning. For example, "type cast": conversion from "int" to "int*_ptr64" of greater size.

  • C4318: Passing zero length. For example, passing constant zero as the length to the memset function.

  • C4319: Not operator. For example, "~": zero extending "unsigned long" to "unsigned _int64" of greater size.

  • C4313: Calling the printf family of functions with conflicting conversion type specifiers and arguments. For example, "printf": "%p" in format string conflicts with argument 2 of type "_int64." Another example is the call printf("%x", pointer_value); this causes a truncation of the upper 32 bits. The correct call is printf("%p", pointer_value).

  • C4244: Same as the existing warning C4242. For example, "return": conversion from "_int64" to "unsigned int," possible loss of data.